User blog comment:Bluetigermonkey11/my theories on where the legends will spawn/@comment-34130872-20180217132133

Like your theories, and I developed some theories by myself.

Tapus & Cosmog evolution line

Maybe in an update, the developers will add an old temple/altar in the Lost Islands-Deep Jungle, like the Altar of the Sunne/Moone. (Maybe Deeper Jungle XD) There you would find the 4 Tapus, crowding over something. Suddenly, Tapu Koko senses you and flees, other tapus also follows, revealing Cosmog. Cosmogs then comes to you, saying it wants to join you, and you are forced to send a poke on your team to the PC. When the cutscene ends, a message would say 'Tapu Koko, Tapu Lele, Tapu Bulu and Tapu Fini are now found roaming in the wild.' However, these pokemon are more easily found than others roaming pokemon because they are neccesary for the next event.

After getting all the Tapus and Cosmog evolved into Cosoem, you should go back to the Altar. Then, another cutscene starts, showing Tapus and Cosmoem bursting from their pokeballs, the Tapus starts 'dancing', restoring the altar into its 'former glory' (all shiny and stuff) and Cosmoem evolves into Sogaleo/Lunala depending on day/night. The cutscene ends.

Necrozma

Should you go back to the altar, a Ultra wormhole appears, and Necrozma approaches you, while you seem interested with it, Sogaleo/Lunala doesn't like it being that close to you, and starts a battle with it. In the end, Sogaleo/Lunala is defeated and is absorbed by Necrozma. Necrozma goes back to the wormhole and disappears. Luckily two strangers appears, and, supprise, its Tess' long lost parents! They explain their long absence, just when the Tapus appears again. It asked them to lend their gift from the Tapus to you, which happens to be a Lunala/Sogaleo, depending which one you already have and is absorbed by Necrozma. Unwillingly, Tess' parents lendit to you, and you started a chase to get Sogaleo/Lunala back.

After you caught up with Necrozma, you challenged a battle with it, it then bursts into Ultra Necrozma and battles you at Lv 70. Note that Tess' parents' Lunala/Sogaleo is useable in place of your stolen Sogaleo/Lunala, and Ultra Necrozma cannot be captured. If you defeated it, Sogaleo/Lunala will seperate from Necrozma, letting you to have a chance to catch Necrozma. After the battle, you return to alter and return the lended Lunala/Sogaleo to Tess' parents.

Ultra Beasts

(This was a hard one, had to think for a while.)

At first, Nilhego would appear in Lost Islands-Deeper Jungle, in a abandoned lake, being a set encounter. Before you able to interact with it though, two familiar people appears, revealing to be 'Beast killers of the Lost Islands' (or something like that). One person actually remembers you because she is the scientist that gave you the Z-crystal pouch! The scientists fully introduce themselves and explain why they are here (because Nilhego, duh). They try to fight it but was overwhelmed. The scientist who gave you the Z-Crystal pouch sees you have already got 6 (or even more) Z-Crystals, and decides to give you some Beast Balls to contain Nilhego. You challenge it into a battle, and before dying/ being captured, it cries out, and the camera is turned into the sky, revealing holes appearing randomly. The other scientists obviously point out that this was not good, and the scientist who gave you Z-Crystal pouch gives you 15 more Beast Balls and gives you a new task: Catch and contain all appearing UBs. Note that the UBs appear randomly in Roria, being a set encounter anywhere. To not to be caught off guard, you can use the previously caught Tapus as a sensor for UBs. If a UB is near, a message will appear:'Tapu XXXX is shaking, a Ultra Beast must be nearby'

Weather Trio

Firstly, you are required to go back to the now-abandnonded Ecilipse Base, where you would find the real Red Orb and Blue Orb in Admin Harry and Gabe's bedroom, respectively. At first the door is locked, but it can be forced opend by Brick Break. Apparently, Team Ecilipse is succesful in duplicating the orbs, allowing them to summon Groudon and Kyogre, but cannot control them. You took the real orbs and challenge Groudon and Kyogre, respectively. After you caught them, you are forced to 'release' both of them together, and a battle between both of them commences, causes Rayquaza to appear and stop them, allowing you to catch them again. Then, Rayquaza take a look at you and challenges you into a battle. Regardless of the result, Raquaza deems you worthy and lets you catch it, making the 1st 100% catch in PBB.

Arceus

Instead of getting Dialga, Palkia and Giratina beforehand, you are required the sacked HM Strength and Hoopa-Unbound in order to battle Arceus. Using Strength, you can get past the massive chain that was blocking the way. Then having Hoopa-Unbound protecting you from the 'toxic gas that would kill you' by being in the first slot of your team, you venture on.

You keep walking until you see a blinding light and suddenly,  Arceus appears, questioning why have you freed the Demon, though it wasn't you. Then, it flew into a rage and attacks you, letting a (unfair) battle commence. Your Hoopa will lose half of its HP as Arceus attacked suddenly. This is a battle that you must lose. After the 1st batle, Dialga, Palkia and Giratina appears, and aids you in battle. This battle, instead of using your normal pokemon, you will use the guardians of time and space and Giratina. After winning, a message appears 'Arceus and Giratina could be found roaming in the wild.'  and end back at the entrance of the tomb.

Reason: Look at the top of the Demon's Tomb, there's the sign of Arceus. Therefore this theory was based on Egytian Mythology: When the sun god Ra/Re retreated to the heavens, he trapped his best and loyal servant Bast (the cat goddess) and his arch-enemy Apep in a prison, letting them fight for eternity. In PBB, Arceus decides to trap himself with the demon and fights it for eternity, but Team ecilipse came and ruined Arceus' plan, freeing the Demon and letting Arceus on his own. In a rage, he blamed the player, who has caught the demon, for his/her own selfish reasons.

Above theories are the ones I think that it would be more reasonable this way, so feel free to give your thoughts.